A storage device includes a nonvolatile memory device and a controller. The nonvolatile memory device includes multiple memory blocks, each of which includes memory cells. The controller reads data from selected memory cells of a memory block selected from the memory blocks during a read operation. The selected memory cells correspond to both a word line and a string selection line selected as a read target. The controller increases a read count by a read weight corresponding to the selected word line and string selection line of the selected memory block, and performs a refresh operation on the selected memory cells if the read count reaches a threshold value. In the selected memory block, two or more read weights are assigned according to locations of the string selection lines and the word lines.